Dual Maelstrom Subs, Port & Alignment Questions..

Discussion in 'Home Theater Projects' started by Mike Keith, Mar 30, 2003.

  1. Mike Keith

    Mike Keith Second Unit

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    My best modeling using the On-Line Parameters shows that a 340Liter box tuned to 20 Hz with 2-4” or one 8” port works best. I have noticed that the response seems to be smoother as the enclosure gets larger up to about 340 Liters then a Peak starts to rise, so based on this I plan using this Volume.


    Maelstrom White Paper used 2-4” X 17” for EBS alignment, however when I model 2-4” ports the length that WinISD gives is no where near this (11.26”) and since I cant change the length as it seems to be Auto Calculated, then I cant look at the difference between the length described in the Maelstrom White Paper. Also will a single 8” port give me problems?

    Right now I’m about to cut a 9.5 foot section of 24” Sono-Tube into 2- 4’ sections for the 340 Liter net pair (actually 355.7 L gross), I want to be sure about my calculations before I cut this piece, and since my modeling doesn’t exactly match what I’ve read, I thought I’d run this by you guys first.


    Any help would be appreciated.
     
  2. Jack Gilvey

    Jack Gilvey Producer

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    Realize that two 4" ports is not the same as one 8" port. Look at the geometry of the two circles. Download WinISD Pro, or Unibox, before you do any cutting. Or just go with the design from Adire.
     
  3. Mike Keith

    Mike Keith Second Unit

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    Yes I do understand that, and I have the latest version of WinISD Pro, it's just that I'm a bit new to it as I normally use Speaker Workshop.

    The reason I asked if there was a problem with a single 8" port as opposed to 2-4" ports is that I've read that to large a port will cause the air inside the port to have it's own resonate frequency, and I'm not sure about what the maximum port velocity should be.

    Also I'm looking for some affirmation on my volume and port size because I can't reproduce the same results as Adire's Maelstrom White Paper.

    340 L with 2-4" X 17" ports for 20Hz Tuning.
     
  4. Pete Mazz

    Pete Mazz Supporting Actor

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    Winspeakerz gives me 2 4" ports 10.5" for a 20 Hz tune in 340 liters. 2 6" would be better at 25.5", if you can fit them.

    Pete
     
  5. Jack Gilvey

    Jack Gilvey Producer

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    Ok, sorry Mike. An 8" port tuned to 20Hz in 340L is only ~22.5" long, and its "organ-pipe" resonance is at 245Hz, well away from the passband of the sub. Should be perfect. With its efficiency, even one of the 250w plate amps will be quite loud.
    Fwiw, I get the same discrepancy with Adire's port lengths for that design. Might want to e-mail Dan.
     
  6. Kyle Richardson

    Kyle Richardson Screenwriter

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    For some odd reason when I built my Maelstrom in 283L using 3ea 4" ports LSPCad (which Adire uses) gave me a much longer port length than the other programs. I used 3 programs and got 3 different lengths so I ended up tuning them by using test tones. It turns out that my desired tuning point needed much shorter lenths that LSPCad said.

    YMMV
     
  7. Mike Keith

    Mike Keith Second Unit

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    Thanks Guy’s for the comments and I must say that I’ve had the same problems with any of the modeling software I’ve used, no two programs will give you the same results.

    That being said I’m back to square one.

    What length and port configuration should I use? Or should I just do as Kyle did and use Test Tones (not sure how to do that though) to determine the length?

    I'm set at 340 gross Liters (less with the ports and driver installed) and I would like to use a single 8” port and shape my own flare into the top plate where the port will be installed, I’ve done this before and it looked and worked quite well, however I wont be able to flare the inside end.

    Any Ideas?

    Also thank's again for the information you guys have been a big help on many occasions.
     
  8. Kyle Richardson

    Kyle Richardson Screenwriter

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    Just start with the longest length the programs say and then run test tones. Where the driver moves the least amount is the tuning frequency of the enclosure.
     
  9. Jack Gilvey

    Jack Gilvey Producer

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    That's the easiest and most accurate way, I think. I use these tones burned on a cd:

    http://terryctheater.tripod.com/15to80sweep.mp3

    It moves slow enough that you can real nail the Fb.

    Even an unflared 8" port would be more than adequate, so don't sweat the lack of flare on the inside.
     
  10. or if you click on my signature, I have 10-100hz in 1/12th octaves in wave format (zip on the link)
     
  11. Mike Keith

    Mike Keith Second Unit

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    Thanks Kyle, Anthony and Jack, that’s the kind of info I'm looking for. I never liked or trusted modeling programs completely and have found that real listening experiments always work best and give me more confidence.

    Now let me see if I got this right, if I want to tune the box to 20Hz then I just play a 20Hz test tone and try different lengths until I get one that will cause the least amount of cone excursion.

    Or;

    What if I sweep the entire low frequency, find the lowest cone excursion, and then check that frequency to see the Fb of that particular length then try different lengths to see the change in Fb.

    Also I have a warble tone generator I could use that goes down to 10Hz maybe that would be a good Idea.


    Thanks again.
     
  12. Jack Gilvey

    Jack Gilvey Producer

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  13. Kyle Richardson

    Kyle Richardson Screenwriter

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    Dont use warble tones and as jack confirms, use the "sweep" method.
     
  14. another thing that may be helpful.

    if you have a DMM, instead of measuring cone excursion, measure the system impedence. The impedence is at a minumum when you are playing a not at the port resonance.
     
  15. Rob Formica

    Rob Formica Stunt Coordinator

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    "sweep the entire low frequency, find the lowest cone excursion, and then check that frequency to see the Fb of that particular length"

    This may seem like a dumb question... but how would you determine the key "frequency" in the middle of the sweep? Interpolating the time elapsed on the sweep track versus the range of the sweep? I'm just not sure I'm using sweeps efficiently...

    Would jumping through single tone sine waves be easier?

    Just wondering... Rob.
     
  16. Jack Gilvey

    Jack Gilvey Producer

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  17. unless you have been inhaling duplicolor spray on liner..then the countingthing gets pretty tough [​IMG]
     
  18. Mike Keith

    Mike Keith Second Unit

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    One of my test CD's has several Test Tones (Alan Parsons & Stephen Courts Test CD) that run together each on a separate track, when it's played it sounds like a Sweep tone that has short breaks in-between when it changes to the next track, so this was what I am planning to use.

    I'm still not sure why the Warble Tone wouldn’t work; maybe Kyle knows something about this.
     
  19. Jack Gilvey

    Jack Gilvey Producer

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  20. Travis Cain

    Travis Cain Stunt Coordinator

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    Let us not forget that WinISD Pro has a tone generator built in.. Very handy..
     

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